Yes, anyone of my customers using IE cannot change mailbox quota.
IE shows that successfully changed the quota to "old value", and still the same!
It will be good if that processed with the next updates for WHM if possible.
We are aware of this issue and have a case for it CPANEL-20622 it's been fixed already in cPanel V74 it looks like we may also be sending the fix to previous versions though that has not been done yet. I'll update here either when that has been done or when 74 is in RELEASE